They are approximately 3.6 billion years old and one-hundredth the diameter of a human hair–but were they ever alive? The tubular structures shown in this photo from a scanning electron microscope resemble in size and shape some tiny micro fossils found in Earth rocks. That observation, along with chemical signatures of organic molecules and mineral features characteristic of biological activity, suggests to NASA's David McKay and colleagues that they may have found evidence of primitive life on Mars. The research by McKay et al. was scheduled to appear in the August 16 issue of Science, but advance reporting of the findings in several major newspapers and a hastily called NASA press conference thrust the issue into the public consciousness. Many scientists, both skeptical and hopeful, anxiously awaited the formal paper. The image is taken from meteorite ALH84001, a rock found in Antarctica but believed to be of Martian origins. (Photo courtesy of NASA).